As the demand increases for construction cranes of the telescopic boom type having increased load lifting capacity, it becomes increasingly necessary to reduce the weight and cost of the boom structure itself without reduction in the strength thereof so as to produce a gain in the net load lifting capacity of the boom and crane.
In a plural section telescoping boom, one of the two reactions of each boom section, except the base section, is applied directly against the top plate of the next outermost or circumscribing boom section by two wear pads arranged near the two side plates of the circumscribing boom section.
The nearer these wear pads are to the side plates, the smaller will be the bending moment induced on the top plate of the circumscribing boom section. The smaller this bending moment, the less the top plate is stressed, thereby allowing the use of a thinner top plate with the desired advantages of lightness and reduction in cost.
By means of the present invention, the pairs of upper wear pads in a typical plural section telescopic boom are held constantly by an adjustable tension resilient means in sliding contact with the side plates of the adjacent circumscribing boom section, in constrast to prior practice where there was clearance between the wear pads and side plates. The upper wear pads utilized in the invention are also in sliding contact with the lower face of the top plate of the circumscribing boom section.
A feature of the invention is that the spring-loading mechanism for the two wear pads is free-floating axially in its support, and this free-floating action allows each inside boom section to move laterally within the circumscribing boom section without disturbing the spring-loading mechanism or interfering with the action of the springs in holding the upper wear pads in contact with the side plates.
Other important features and advantages of the invention will become apparent during the course of the following description.